Liquid crystals are spontaneously ordered fluids characterized by a uniaxial, lamellar, helical, or columnar arrangement in nematic, smectic, cholesteric, or discotic mesophase, respectively. By preserving these molecular arrangements in the solid state via cooling through glass transition temperature (Tg), glassy liquid crystals (GLCs) represent a unique material class potentially useful for organic optoelectronics. Whereas all liquids are expected to vitrify at a sufficiently rapid cooling rate, most organic materials, including liquid crystals, tend to crystallize upon cooling through the melting point, Tm. Crystallization of liquid crystals essentially destroys the desired molecular order that prevails in the fluid state, resulting in polycrystalline films that scatter light or impede charge transport.
The first known or reported attempt to synthesize GLCs in 1971 yielded materials with a low Tg and poor morphological stability, namely, the tendency to crystallize from the glassy state. Subsequent efforts have produced GLCs that can be categorized into (i) laterally or terminally branched, one-string compounds with a Tg mostly around room temperature; (ii) twin molecules with an above-ambient Tg but generally lacking morphological stability; (iii) cyclosiloxanes functionalized with mesogenic and chiral pendants; (iv) carbosilane dendrimers exhibiting a low Tg; (v) macrocarbocycles with mesogenic segments as part of the ring structure; and (vi) pentaerythritol as the central core to yield widely varying Tg and morphological stability. In particular, cholesteric GLCs are potentially useful as large area non-absorbing polarizers, optical notch filters and reflectors, and polarizing fluorescent films. Moreover, cholesteric GLC films can serve as a one-dimensional photonic bandgap for circularly polarized lasing. Comprising separate chiral and nematic pendants, cholesteric GLCs have been synthesized either by a statistical approach, which requires intensive workup procedures to arrive at pure components, or by deterministic approaches, which require long synthesis schemes.
Cholesteric GLCs with hybrid pendants having both chiral and nematic moieties chemically bonded to a volume-excluding core addresses the problems of complexity and cost associated with previous cholesteric GLC systems with separate chiral and nematic pendants. However, previous attempts at hybrid pendants have met with little or no success. For example, hybrid pendants with a chiral tail yielded exclusively smectic mesomorphism (Delavier et al., U.S. Pat. No. 5,840,097), and cyanotolan with a chiral spacer to a cyclohexane core failed to achieve mesomorphism (Shi et al., Liq. Cryst., 1994, 17, 413).
Based on the foregoing, there is an ongoing unmet need for cost effective preparation of cholesteric glassy liquid crystals with elevated phase transition temperatures, stability against crystallization from the glassy state, and selective reflection across the visible to near infrared region.